(1) Field of the Invention
The present invention relates to an electro-mechanical energy harvesting device and more particularly relates to such a device that has a magnetostrictive and piezoelectric component.
(2) Description of the Prior Art
It is known that ferroelectric single crystals convert mechanical energy to electrical energy or vice versa. This makes them a candidate as the active material in energy harvesting devices. By utilizing the direct piezoelectric (or pyroelectric) effect when mechanical or thermal energy is available from the environment, the mechanical or thermal energy can be converted to electric charge polarization in relaxor ferroelectric single crystal material and useful amounts of energy can be obtained.
Relaxor single crystals display both a linear piezoelectric effect and a non-linear electromechanically coupled phase transition. The linear piezoelectric effect in relaxor single crystals has been well characterized and is extraordinarly large, approximately a factor of six times that of the ceramic lead zirconate titanate (PZT). The non-linear electromechanically coupled phase transition associated with field and stress driven phase transformations has been the subject of extensive study, especially for lead indium niobate-lead magnesium niobate-lead titanate (PIN-PMN-PT) ternaries. Reversible stress and temperature induced phase transformations are associated with spontaneous charge generation in the relaxor single crystals. FIG. 1 is a graph showing strain versus stress for a representative phase change piezoelectric material. FIG. 1 clearly shows a large strain jump at the stress and field induced phase transformation. These stress and field driven phase transformations offer significant new approaches to energy harvesting. These results demonstrate that phase transformations can provide more than an order of magnitude increase in energy density per cycle for mechanical energy harvesting. Utilizing this phase transformation behavior suggests that a stress-biased energy harvester would take maximum advantage of the phase transformation in the relaxor single crystal material.
Magnetostrictive materials are similar to ferroelectric materials because they convert magnetic energy into mechanical energy. However, magnetostrictive materials utilize a magnetic field rather than an electrical field. An applied magnetic field can alter the direction of the magnetic moments inside the material, and the magnetic moments will tend to align themselves in the direction of the applied magnetic field. This directional change of the magnetic moments is coupled to the material's lattice via spin-orbit coupling and results in a physical change in the dimension of the material. It is known to utilize this physical change in mechanical applications and control systems.
It is known to use piezoelectric materials to harvest energy. Piezoelectric materials will generate electric potential when subjected to some kind of mechanical stress. However, piezoelectric materials have constraints on their ability to function properly, such as temperature, force, and pressure. These constraints, along with the difficulty of attaching piezoelectric materials to rotating or moving machinery, make it difficult to locate piezoelectric material devices in contact with the mechanical stress generator which allows the piezoelectric material to act as an energy harvesting device.
It is also known to use magnetostrictive materials to harvest energy. Magnetostrictive materials are able to harvest vibrational energy from vibrating pumps, motors, buildings, ships, etc. since magnetostrictive materials are able to change shape in response to a magnetic field, and it is known to use these changes in magnetic state to induce a voltage in coils, which can then be converted into power.